This invention relates to improvements in focus actuators, i.e., devices used to control the movement of a lens in a direction parallel to its optical axis to maintain a beam of radiation passing through the lens in sharp focus on a surface moving toward and away from the lens. The present invention is particularly useful in the fields of optical and magneto-optical recording where it is necessary to maintain a read/write laser beam in sharp focus on a spinning disk-shaped recording element.
It is well known in the art how to control the focal position of an objective lens in optical and magneto-optical recording systems with moving coil, moving magnet or moving armature focus actuators. See, for example, the focus actuators disclosed in U.S. Pat. Nos. 4,302,830 and 4,669,823. Such actuators typically comprise one or more permanent magnets which serve to produce a constant magnetic field which, together with a variable magnetic field produced by a varying electrical current (as produced by a focus servo system), controls the lens position relative to a niminal set point. While capable of providing the unerring and precise magnetic field required by high performance actuators, permanent magnets present certain disadvantages. For example, such magnets are expensive and require precision grinding and careful mounting due to their hard and brittle nature. Also, owing to their relatively large mass, moving permanent magnets place an added axial load on the springs used to resiliently mount the objective lens for movement about a nominal set point; this added load requires greater stiffness of the spring and can adversely affect the servo function. Moreover, the continuous magnetic flux between the magnet's pole pieces tends to attract small particles. Such particles are difficult to remove and can eventually alter the response characteristics of the actuator.